1. Field of the Invention
The present invention relates generally to a Hybrid Automatic Repeat reQuest (HARQ) technology in a mobile communication system, and in particular, to an HARQ method in a mobile communication system, capable of improving a data rate by minimizing use of resources, and a transmission/reception method and apparatus using the same.
2. Description of the Related Art
Generally, multiple access schemes of mobile communication systems are classified into Frequency Division Multiple Access (FDMA) in which a predetermined frequency band is divided into a plurality of channels and users use frequency channels uniquely allocated to them, Time Division Multiple Access (TDMA) in which one frequency channel is time-shared by a plurality of users (or subscribers), and Code Division Multiple Access (CDMA) in which a plurality of subscribers use the same frequency band at the same time but they perform communication using different codes uniquely allocated to them. With the rapid progress of communication technology, the mobile communication systems have reached the phase of providing, not only, the general voice communication service, but also, a packet data (packet) service in which users can perform high-speed, high-quality digital data transmission and multimedia services with a mobile terminal.
Today, in mobile communication systems supporting the packet service, HARQ technology is generalized in which for stable packet transmission, a mobile terminal sends, to a base station, information indicating success/failure in packet reception upon receipt of a packet from the base station, and then the base station retransmits the defective packet that the mobile terminal has failed to normally receive. HARQ is a kind of link control protocol in which upon receipt of a defective packet, a mobile terminal sends a base station a request for retransmission of the corresponding packet. Generally, it is actually impossible for a mobile terminal of the mobile communication system to receive the packets transmitted via a wireless network without any distortion or noises. Therefore, the HARQ technology presents various packet retransmission techniques to solve these problems.
HARQ technology-related terms used herein will be defined below.
The term ‘packet’ refers to a bit stream generated after encoding the original information, and the term ‘subpacket’ refers to a lump of bits transmitted at once when a coded bit stream is transmitted after it is divided into a plurality of lumps each composed of consecutive bits. In addition, ‘control information’ refers to addition information other than the original information necessary for receiving the subpacket or packet. Further, the term ‘initial transmission subpacket (or initially transmitted subpacket)’ refers to a first transmitted subpacket among subpackets, and the term ‘retransmission subpacket (or retransmitted subpacket)’ refers to a subpacket transmitted after the first subpacket.
The HARQ technology can be roughly classified into Synchronous HARQ (S-HARQ) and Asynchronous HARQ (AS-HARQ). S-HARQ is characterized in that retransmission for initial transmission is performed within a predetermined time. Herein, ‘Synchronous’ means synchronization in the time domain. AS-HARQ is characterized in that a time interval between the initial transmission and the retransmission is not fixed.
In S-HARQ, because a transmission time is predetermined, a retransmission subpacket should always be transmitted regardless of the channel condition, increasing the possibility of packet loss. However, in AS-HARQ, a receiver should first determine whether a received subpacket is a subpacket transmitted to the receiver itself. Therefore, even though the receiver has succeeded in reception of the initial transmission subpacket, it cannot always guarantee reception of the retransmission subpacket. However, AS-HARQ can select a time having a good channel condition for the retransmission, because the time interval between the initial transmission and the retransmission is not fixed.
Because AS-HARQ can determine a transmission time according to the channel condition, it can cope with a change in the channel condition in the manner of changing a modulation scheme at every retransmission time. However, it needs additional control information. S-HARQ can also use the channel adaptation scheme like the method of changing the modulation scheme, as it transmits control information at every retransmission time. However, given that S-HARQ is advantageous over AS-HARQ in that it can minimize the control information, the method of transmitting the control information at every retransmission time is not an effective approach.
Currently, FDMA systems are under discussion together with a resource allocation scheme and an HARQ scheme. The FDMA system is different from the conventional CDMA system and other communication systems in that it can define a basic unit of the resource allocation not only in the time domain but also in the frequency domain. In the conventional CDMA system, because user signals multiplexed with codes are transmitted over the full frequency band, resource allocation in the frequency domain is impossible. However, in a communication system, like the FDMA system, in which the basic resource allocation unit is defined not only in the time domain but also in the frequency domain, there is a need to modify the HARQ technology, which is based on the conventional CDMA system, to be suitable for the corresponding communication system. This is because intact use of the conventional HARQ technology may prevent efficient resource allocation (i.e., scheduling).
A typical example of the FDMA system, recently attracting attention, includes an Orthogonal Frequency Division Multiple Access (OFDMA) system. In brief, the OFDMA system refers to a multiple access system based on Orthogonal Frequency Division Multiplexing (OFDM), which is a multi-carrier transmission scheme that transmits a plurality of orthogonal subcarriers in an overlapping manner. OFDM can be applied to digital transmission technologies such as Digital Audio Broadcasting (DAB), Digital Television, Wireless Local Area Network (WLAN), etc. and it is known that OFDM can provide an efficient platform for high-speed data transmission due to its advantage of being robust against multipath fading.
When a forward link of a system capable of transmitting a plurality of channels over one slot, including the foregoing OFDMA system, uses the HARQ technology, resource allocation can be classified into persistent resource allocation for persistent traffic and non-persistent resource allocation for non-persistent traffic according to traffic type.
The persistent traffic means the traffic having a data characteristic in which the desired amount of transmission data can be persistently expected for a long time, like the Voice over Internet Protocol (VoIP) or Video Telephony data. However, the non-persistent traffic means the traffic whose transmission time and amount can be hardly expected, like the http, FTP, and e-mail data. A non-persistent resource allocation scheme for supporting the non-persistent traffic, as it allocates resources only when there is transmission data, transmits the traffic maximally taking the channel condition into account, pursuing high spectrum efficiency. On the contrary, a persistent resource allocation scheme for supporting the persistent traffic can minimize the amount of resources required for transmission of control information, because a transmitter and a receiver are allocated a predetermined amount of resources at a predetermined time without performing a separate signaling procedure between them. However, the persistent resource allocation scheme has difficulty in adaptively transmitting retransmission data by selecting a good channel environment.
Therefore, when a mobile communication system, like the OFDMA system in which resource allocation and non-persistent resource allocation may coexist, uses the HARQ technology, there is a need for a scheme capable of efficiently performing resource allocation.